A frequently applied principle of anesthetic dispensing is based on the separate supply of anesthetic gases and fresh gas to a mixing site. The gaseous components are sent here through a pipe system, in which various flow resistances are integrated. The fresh gas is fed to the mixing site via a line in which a fixed flow resistance, a so-called bypass, is integrated. The anesthetic gas is fed via a line, in which a variable flow resistance, which is used to dispense the anesthetic gas, is integrated. The two lines meet at the mixing site. The mixing of the gaseous components takes place there. The gas mixture is fed to the patient away from the mixing site.
The anesthetic gas is dispensed by varying the variable flow resistance of the dispensing device. A constant mixing ratio of the anesthetic gas to the fresh gas is desirable here. Such a system is therefore usually calibrated such that in case of correct dispensing of the anesthetic, the same pressure prevails in front of the two flow resistances, namely, the bypass and the variable flow resistance of the dispensing device.
The maintenance of this state is monitored by a differential pressure sensor, which detects the pressure in both lines. The measured differential pressure equals zero in the ideal case. However, very low pressure values are maintained in the normal case of operation.
The sensitivity of the differential pressure sensor used for this differential pressure measurement is therefore usually very high, which leads to a narrow measuring range at very good accuracy of measurement. Highly accurate monitoring of the mixing ratios can thus be guaranteed. One drawback of such a device is that suddenly occurring higher differential pressures may lastingly damage a differential pressure sensor designed for such a sensitivity when its measuring range is exceeded.
A stable state of flow becomes established in the system of supply lines supplying the gaseous components to the mixing site in case of normal operation. A negligibly low differential pressure becomes established at the differential pressure sensor in this stable state of flow.
If there is an undesired disturbance of this stable state of flow for various reasons, the individual branches of the supply lines are not able, because of the different flow resistances integrated in them, to respond to these changes or disturbances with the same time constant. A new state of flow will consequently become established with different delays in the individual branches of the line. Differential pressures, which exceed the measuring range of the differential pressure sensor by several times, may develop at the differential pressure sensor during this setting time. A long-lasting difference in the pressures present is, by contrast, not possible, because the supply lines are connected with one another at the mixing site and thus form a pneumatic short-circuit.
However, the brief occurrence of high differential pressures is sufficient to inflict lasting damage to the sensitive differential pressure sensor. It is possible, for example, that the connection in the direction of the patient will be briefly closed. This leads to a rapid rise in the pressure in the system, which can maximally correspond to the pressure of the fresh gas supply system. If the system is opened again, an equally rapid release will occur. These dynamic processes do not lead to equal changes in pressure at the same time at the two points at which pressure is tapped, at which the differential pressure sensor is connected with the lines for supplying the gaseous component, because of the differences in the flow resistances of the bypass and the variable flow resistance. Consequently, great differential pressures will build up for a short time.